1. Field of the Invention
The present invention relates to a polycrystalline diamond composite compact for use in drilling operations which require high wear resistance of a diamond surface. More specifically, the present invention relates to a polycrystalline diamond layer attached to a cemented metal carbide structure used as a cutter in a drill bit for drilling operations wherein the cutter has improved toughness or fracture resistance during use.
2. State of the Art
Polycrystalline diamond tools suitable for use in rock drilling operations are well known. Typically, the polycrystalline diamond cutters used on such tools are composite compacts comprising a polycrystaline diamond layer and a cemented carbide support structure. Typically, the carbide support structure comprises tungsten carbide containing cobalt metal as the cementing constituent. The cobalt contained in the carbide support structure functions as the bonding metal for the carbide, as a sintering aid for consolidating the diamond particles into a solid attached diamond layer, and to bond the diamond layer to the carbide support. Care must be exercised regarding the amount of cobalt used as an excessive amount of cobalt infiltrated from the carbide support structure into the diamond layer leaves an excessive amount of cobalt among the diamond particles, thereby affecting the mechanical properties, possibly causing less than optimal abrasion resistance of the diamond layer. Also, the physical and mechanical properties of the cemented carbide support structure near the diamond/carbide interface are affected as a result of the cobalt depletion from the carbide support. Typically, the cobalt depletion of the carbide support structure adjacent to the interface results in reduced mechanical properties in a critical area of the diamond tungsten carbide cutter.
Various methods are used to control the cobalt infiltration into the diamond to prevent excessive infiltration into such layer and the attendant cobalt depletion of the carbide support structure. Typical prior art diamond cutters are described in U.S. Pat. Nos. 4,988,421; 5,011,514; 5,011,515; 5,022,894; 5,111,895; 5,151,107 and 5,176,720 as well as European Patent Application 0,246,789.
Also, attempts have been made to increase the hardness of cemented carbide bodies, which bodies include a tungsten backing of the polycrystalline diamond compact, and are made by sintering pressed carbide powders to provide cutting implements having the ability to hold a sharper edge or longer life. Such cemented carbide bodies typically are comprised of a mixture of tungsten carbide and cobalt. Typically, in forming such bodies, a trade-off occurs between brittleness and hardness. The harder the body is, the better the body holds a cutting edge; However, the more brittle the body.
One attempt to avoid the increased brittleness while improving hardness has been to produce a thin surface coating or layer on the carbide body containing boron by diffusing boron into the surface of the cemented carbide body. However, as the thin coating is worn away, the improved properties of hardness as well as other features are lost. Another attempt has been made to improve the properties of a cemented carbide body made by sintering pressed carbide powders in the presence of boron containing material to diffuse the boron to a greater depth in the cemented carbide body. Such cemented carbide bodies are described in U.S. Pat. Nos. 4,961,780 and 5,116,416. These types of cemented carbide bodies including boron show improved fracture toughness over bodies which contain no boron.